Many modern-day aircraft include leading edge high lift devices (e.g., slats and Krueger flaps) to improve performance during certain phases of flight (e.g., takeoff and landing). Many of these aircraft also have wing-mounted propulsion units that are carried under the wing of the aircraft. The wing-mounted propulsion units can interfere with flow over the wing and detract from aircraft performance.
FIG. 1A is a partially schematic top plan view and FIG. 1B is a partially schematic front elevation view of an aircraft wing 10 having a wing-mounted propulsion unit 20 in accordance with the prior art. The wing 10 can include an inboard high lift leading edge device 12a and an outboard high lift leading edge device 12b. The propulsion unit 20 can include an engine 26, positioned within a housing 22, which is supported by a pylon 21. The propulsion unit 20 can be coupled to the wing 10 via the pylon 21 so that at least a portion of the propulsion unit 20, including the pylon 21, extends forward from the leading edge 11 of the wing 10. Accordingly, there can be a gap 5 between a portion of the housing 22 and a portion of the wing 10. This gap 5 can be a major contributor to the flow interference created by the wing-mounted propulsion unit arrangement.
The local flow pattern around and through the gap 5 can be quite complex, especially when the aircraft is operated at higher angles of attack and/or when the high lift leading edge devices 12a, 12b are deployed. The interface between the wing 10 and propulsion unit 20, including the gap 5, is often optimized for the cruise condition when the high lift leading edge devices 12a, 12b are stowed. As angle of attack is increased above cruise values, the air flow through the gap 5 can often cause the performance (e.g., local drag coefficient and/or local lift coefficient) of the local wing section proximate to the gap 5 to degrade as compared to similar wing sections located away from the gap 5. Additionally, because the propulsion unit 20 interferes with the installation and/or deployment of high lift leading edge devices, the high lift leading edge devices 12a, 12b often do not extend laterally across the wing 10 in the area where the gap 5 is located. Accordingly, the local wing section proximate to the gap 5 does not get the performance benefits provided by the high lift leading edge devices 12a, 12b. Even if the high lift leading edge devices 12a, 12b extend laterally across the gap 5, the flow interference created by the wing 10 and the propulsion unit 20 can reduce the performance of the local wing section proximate to the gap 5.
One approach to addressing this problem is to provide the aircraft with a small flap-type device 30 that seals the gap when the high lift devices 12a, 12b are deployed. In FIG. 1, the flap-type device 30 is shown in the sealed position by dashed lines. In the sealed position, the flap-type device 30 extends from the bottom portion of the wing 10 and seals against a portion of the engine housing 22. Additionally, if the housing 22 moves with the deployment of a thrust reverser, the flap-type device 30 or a portion of the flap-type device 30 can be configured to move with the housing 22 to maintain the sealed position. The flap-type device 30 can also have a retracted position in which it is positioned proximate to the bottom of the wing 10 during cruise when the high lift leading edge devices 12a, 12b are retracted. In the sealed position, the flap-type device prevents air from flowing through the gap 5, reducing drag and/or increasing a lift/drag ratio during certain phases of flight (e.g., during takeoff). A drawback of this arrangement is that the drag and/or lift profile of the local portion of the wing 10 proximate to the gap 5 can still be less than desirable for selected phases of flight conducted with the high lift leading edge devices 12a, 12b deployed. For example, during a landing phase of flight it can be more desirable to increase a maximum lift coefficient than to minimize drag, requiring a different configuration than is provided by the flap-type device 30 in either the retracted position or the sealed position.